Method for automatically adjusting the aeration of an installation for biological treatment of waste water

ABSTRACT

The oxidoreduction potential is measured on each of a number of tanks. A data processing system analyzes the oxidoreduction potential evolution and establishes a diagnosis of the aeration/biology couple to ensure the degradation reactions of the carbonaceous and nitrogenous pollution. It is then determined whether to start, to continue, or to stop aeration in the tanks, depending on the required degree of purification. In the first tanks, priority is given to the treatment of carbonaceous pollution with specific parameter representation of the oxidoreduction potential threshold values. In the last tank, fine-tuning the treatment of the carbonaceous and nitrogenous pollution is accomplished by managing the aeration.

FIELD OF THE INVENTION

The invention relates to a process for e automatic regulation of theaeration of a biologic MAC wastewater treatment plant for the removal ofcarbon and nitrogen pollution.

BACKGROUND OF THE INVENTION

It is known that the purification of wastewater constitutes a majorproblem. Accordingly, the European Union has been led to issue adirective (No. 91/271/EEC) relating to the treatment of urban wastewaterwhich determines the limits of discharges, into the natural environment,of untreated wastewater. Thus, each treatment unit is attributed aprecise objective as regards the quality of the water treatment, itbeing possible for failure to achieve such an objective to give rise topenalties of a financial or even a penal nature.

The majority of urban wastewater treatment plants use the activatedsludge process. An important phase of this process consists in theremoval of the carbon and the nitrogen contained in the wastewater, bysequencing of the periods of aeration. It is indeed known that the mainproblem encountered in wastewater treatment plants is adapting thetreatment to the variations in the rate of entry of the water to bepurified and to its polluting load, so as to obtain a constant qualityof purified water and the minimum regulatory quantity of pollutingdischarges into the natural environment. For this purpose, the removalof carbon and nitrogen requires a very strict and precise control ofaeration given that this removal must correspond to two requirements.According to the first, a sufficient total duration of aeration shouldbe provided per day in order to carry out the oxidation of the carboncomponents of the wastewater and the stabilization of the sludge; thesecond is linked more directly to the daily distribution of the aerationphases in order to successfully carry out the removal of the nitrogen.On the one hand, it is necessary to observe a sufficient period formaintaining under aerobic conditions for the sludge to perform thenitrification and, on the other hand, the nitrification requires anappropriate effluent residence time under anoxic conditions. For thispurpose, in the small-load activated sludge processes used in a singleaeration basin, the removal of nitrogen compounds results from a strictcontrol of the alternation of the aerated and nonaerated sequences.

Any defect in the setting or the operation of the oxygen supply devicesresults in a malfunction of the wastewater purification stations, withrepercussions on the quality of the effluent treated, the equilibrium ofthe purifying biomass and the characteristics of the sludge produced.

A lack of adaptation of the aeration sequences therefore has effects inthe short term on the quality of the water obtained which may thencontain nonoxidized nitrogen compounds if the periods of aeration arenot sufficiently long, or nitrates if the periods of anoxia are tooshort. By contrast, when the periods of nonaeration are too long, theeffluent to be treated encounters anaerobic conditions which must beabsolutely avoided. Indeed, the phenomena of anaerobiosis in thetreatment basin, linked to an under-oxygenation of certain zones, causein the long term the appearance of filamentous bacteria and thesemicroorganisms induce a modification of the structure of the floc and areduction in its sedimentation ability, which of course has anunfavourable repercussion on the quality and the cost of the treatment.Another consequence of an insufficient cumulative duration of aerationrelates to the quality of the sludge and, in particular, determines itsstability.

It can be understood why the regulation of aeration is one of the keypoints in such a water treatment process. Various methods of regulationhave been used.

Thus, sensors measuring dissolved oxygen and the oxidation-reductionpotential as well as various sensors serving to detect reference valueshave been used, a high threshold making it possible to stop the aerationand a low threshold to restart the aeration system, delaying means beingused when the thresholds are not reached. These known systems are notcompletely satisfactory. Indeed, to optimize the nitrification anddenitrification reactions, it is essential to supply oxygen whennecessary and in a sufficient quantity, and not simply to supply oxygenas done by the systems described above.

In order to improve these systems, it has been necessary to continuouslymeasure the oxidation-reduction potential EH of the medium and toanalyse the go shape of the curve of the variation of this potential asa function of time. According to this known process, the derivative ofthis function EH=f(time) is calculated. If this derivative is negative,this corresponds to a reduction in the oxidation-reduction potential inanoxia phase and when the derivative is zero, a stabilizing phase ispresent. The system then calculates the period of aeration or ofnonaeration to be allowed for, which is equal to the period of aerationor of nonaeration necessary to bring the oxidation-reduction potentialto the value required to perform either the removal of carbon, or thenitrification, or the denitrification, plus the additional timenecessary to perform the reaction. Such a system has disadvantagesbecause the oxidation-reduction potential curve can take on anasymptotic shape (derivative tending towards 0) and, for certainoxidation-reduction potential values, it is absolutely necessary toavoid passing to a virtually stabilizing phase because the conditionsare then inappropriate for the desired treatment (poor preservation ofthe biomass).

The present proprietor has been led to perfect the latter process. Thus,its patent FR-A-2,724,646 describes a system for regulating the aerationof a biological wastewater treatment using an activated sludge treatmentapplied to a combined removal of the carbon and of the nitrogen in asingle basin plant provided with aeration means. Such a systemrecognizes, in real time, the level of treatment required in the basinand it controls the appropriate aeration sequence. Moreover, it providesa diagnostic assistance as regards the possible limits of the process.Thus, the aeration is better adapted to the conditions of the processand the reliability and the energy and economic management of theaeration are improved. It is thus possible to obtain complete removal ofthe carbon and of the nitrogen while maintaining a sufficient oxidationstate of the sludge.

However, this system of regulation has limits. While it is perfectlysuitable for activated sludge purification processes of the small loadtype, with a single and homogeneous aeration basin, it cannot be used ontreatment sites whose configuration leads to several distinct aerationvolumes or basins, placed in series, being differentiated. FIGS. 1a and1 b of the appended drawings very schematically represent two examplesof such treatment plants. The cells of the aeration basins beingphysically separated, the oxidation state and the progress of thebiological reactions in the basins are no longer uniform in the wholeaeration volume. An alternation of the aerated and nonaerated sequencesis still necessary in order to carry out the reactions. The knownprocess, which was last to be described above, is not directlyapplicable to nonhomogeneous aeration basin configurations in series,given that the representativeness of the measurements of theoxidation-reduction potential cannot be ensured for the entire aerationvolume of such basin configurations. In addition, a minimum residualcarbon content is necessary in order to maintain the kinetics ofdenitrification at a high level. In case identical aeration sequencesare maintained for all the aeration cells, the heterogeneity of the rateof progress of the reactions does not make it possible to successfullycarry out and to optimize the biological reactions. It is thereforenecessary to manage the aeration of the different basins in anindependent and complementary manner. This is what constitutes thetechnical problem which is solved by the present invention.

BRIEF DESCRIPTION OF THE INVENTION

The subject of the present invention is therefore a process forregulating the aeration of a biological wastewater treatment plant, forthe purpose of removing the carbon and nitrogen pollution, comprisingseveral aeration basins arranged in series, this process beingcharacterized by the following stages:

the oxidation-reduction potential EH is measured separately on each ofthe basins;

the said measurements are respectively transmitted to a data processingsystem for the purpose of analysing the variation of theoxidation-reduction potential and the establishment of the capacities(diagnoses) of the aeration-biology pair to bring about the reactionsfor degrading the carbon and nitrogen pollution,

the starting, the continuation or the stopping of the aeration of thesaid basins is determined, from the said diagnoses, as a function of thedesired degree of purification such that:

in the first basins, or upstream basins, the priority is given to thetreatment of the carbon pollution with a specific parameterizing of thethreshold values of the oxidation-reduction potential,

in the last basins, or downstream basins, the treatment of the carbonand nitrogen pollution is refined by a management of the aeration inaccordance with the process described in FR-A-2, 724, 646.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1 b respectively represent serial and parallel connectedbasins, as known in the prior art;

FIG. 2a is a basic flowchart of a multiple basin configuration of seriesconnected basins as employed in the present invention;

FIG. 2b is a basic flowchart of a U-shaped basin as employed in thepresent invention;

FIG. 3a is a schematic indication showing the relationships between theoxidation-reduction potentials and the objectives of biologicalreactions carried out in upstream cells;

FIG. 3b is a schematic indication showing the relationships between theoxidation-reduction potentials and the objectives of biologicalreactions carried out in downstream cells;

FIG. 4 is a representation of the oxidation-reduction potential (EH) asdivided into a succession of zones; and

FIG. 5 is a plot that illustrates the effects of a modification of apollution regime on an aeration sequence.

DETAILED DESCRIPTION OF THE INVENTION

As can be understood, the technical problem solved by the presentinvention consists in bringing about an independent but complementaryregulation in the different aeration cells of the plant.Oxidation-reduction potential detectors are therefore positioned on eachdistinct fraction of the aeration volume. FIGS. 2a and 2 b of theappended drawings very schematically represent multiple basinconfigurations in series: separate basins (FIG. 2a) and U-shaped basin(FIG. 2b). In the case of the U-shaped basin, provision is made for twoseparate aeration control systems for bringing about regulation on eachof the aeration cells which are delimited in this particular case. Aseparate regulation of the aeration is performed on the upstream anddownstream cells. According to the invention, the progress of thenitrification reactions is deliberately limited on the upstream cells,by limiting the length of the aeration sequences, the priority beinggiven to the removal of most of the carbon with the existence of asufficient leakage of the residual carbon compounds towards thedownstream cells in order to promote the denitrification therein. Thus,the downstream cells bring about complete removal of the nitrogenpollution and complete the removal of the remaining carbon. According tothe invention, complementarity of the upstream and downstream treatmentsis provided, with a completion of the downstream treatment and areduction in the load variations by the upstream cells.

FIGS. 3a and 3 b illustrate, respectively, the relationships between theoxidation-reduction potentials and the objectives of the biologicalreactions carried out on the upstream and downstream cells,respectively.

According to the process of the invention, the upstream cells aremanaged by a suitable logic which gives priority to the removal of thecarbon pollution with a specific parameterizing of the threshold valuesof the oxidation-reduction potential, whereas the downstream cells aremanaged in accordance with the method of regulation described inFR-A-2,724,646, which makes it possible to finish the treatment by acomplete removal of the nitrogen components and of the carbon componentsremaining after the treatment carried out in the upstream cells. Duringthe aeration phases, the oxidation-reduction potential provides thedesired degree of oxidation of the sludge. Thus, on the same automaticlogic bases, the regulations of the aeration cells are distinct.

As can be understood, the programming is not based on a variationbetween thresholds, as in traditional automatic devices, but on theanalysis of the variation of the curve EH=f(time).

As in the abovementioned French patent, the regulation is based on theanalysis of the variations of the oxidation-reduction potential EH,these variations being representative of the changes in state of thespecies present: which changes can occur depending on the load for thepurification station, the quality of the effluent to be treated enteringthe station, the temperature and the like.

The data processing system makes it possible to analyse the variation ofthe oxidation-reduction potential by calculating the derivatives(variation of the measurement over an interval of time), as in thealready cited French patent. This analysis makes it possible to definethe following variations:

positive derivative=increase in the concentrations of oxidized forms;

negative derivative=increase in the concentrations of reduced forms;

zero derivative=stabilization phase.

When the derivative tends towards the immeasurably small, a study ofthreshold variation is carried out which makes it possible to know thedirection of variation of the oxidation-reduction potential EH. Therange of oxidation-reduction potentials is divided into a succession ofzones defining a quality criterion. As can be seen in FIG. 4 of theappended drawings and as already specified in the abovementioned Frenchpatent, the following are identified:

3 activity or “quality criterion” zones:

criterion 1 zone: nitrification zone

criterion 2 zone: zone for removing the carbon and for partialdenitrification

criterion 3 zone: denitrification zone;

2 danger zones:

a warning zone, in which the potential EH should not be maintainedbecause it corresponds, in aeration phase, to operating conditions forwhich the oxygen content is insufficient to ensure removal of thecarbon, and, in nonaeration phase, to an excessively high residualoxygen content which is incompatible with the denitrification;

a forbidden zone, in which the risks of severe anaerobiosis areconsiderable and risk bringing about a major malfunctioning of thestation.

Following its analysis (rate of variation and “quality criterion” zone),the automatic device establishes a diagnosis of the capacity of theaeration/biology pair to bring about the reactions for degrading thenitrogen and carbon pollution.

After establishment of the diagnosis, the automatic device determinesthe optimum periods of operation or stoppage of the aeration systemsdepending on the desired degree of purification, the regulations of theaeration cells being distinct on the same automatic logic bases:

period of aeration necessary to reequilibrate the system in order toreestablish a satisfactory degree of oxidation of the sludge;

period of aeration necessary for an oxygen supply which allows the bestuse of the reactions for degrading the carbon load in the upstreamcells;

period of aeration necessary to complete the supply of oxygen in orderto optimize the reactions for degrading the carbon load and thenitrification in the downstream cells;

period necessary for stopping aeration in order to complete thedenitrification in the downstream cells.

Apart from the actual regulation on the upstream and downstream aerationcells, the automatic device can manage warnings or defects when a humanintervention is necessary, which makes it possible to detectdiscrepancies in the information received and to make them known to theoperators, thereby improving the reliability of the aeration process asa whole.

There are three main causes likely to trigger the warning functions ofthe regulation system according to the invention:

a) major process disorders, such as failure of aeration means, apermanent pollution overload or even a deterioration in the biomass. Inthis case, the sole mission of the regulation system is to report theincidents to the operators;

b) metrological problems and, in particular, problems of validation ofthe oxidation-reduction potential measurement signal: defects in probesfor measuring potential or maintenance inadequacies then cause a swingin the management of aeration on a “time” programmer, so as to free theregulation of the said aeration with respect to the measurement of theoxidation-reduction potential;

c) secondary warnings may also be activated when minor incidents appear,for example a temporary pollution overload or hydraulic interactionsbetween the aeration cells. While bringing about the regulation ofaeration, the system according to the invention warns the operator ofthese incidents.

The curves of the variation of the oxidation-reduction potential as afunction of time in the system for automatic regulation of the aerationof the upstream and downstream cells have been represented in FIG. 5 ofthe appended drawings. They are of the same type as those which areobserved in the case of the regulation on a single aeration basin,according to the already cited French patent, but they have,nevertheless, some clearly distinct features resulting from thedifferent functions assigned to the different cells.

On the downstream aeration cells (curve of variation of the potential EHas a fine line), the domain of the variations of the oxidation-reductionpotential signal is similar to that observed on an aeration cell inaccordance with the prior art process described in the above citedFrench patent.

On the upstream aeration cells (curve of variation of the potential EHas a thick line), the domain of variations of oxidation-reductionpotential is deliberately reduced in order to limit the occurrence ofthe nitrification reactions: this results in an increase in thefrequency of the aeration sequences. As a guide, the daily frequency ofthe aeration cycles is greater than 15 on the upstream cells bringingabout, as a priority, the removal of the carbon pollution, whereas thisfrequency is of the order of 7 to 12 daily aeration cycles for thedownstream cells, bringing about the treatment of the nitrogenpollution.

On the upstream cells, the envelope of the curve of variation of theoxidation-reduction potential is representative of the diurnal supply ofpolluting load. During the night, the amplitudes of the variations arereduced and the increases in the oxidation-reduction potential areslower. This reduction in amplitude is less marked on the signalsobtained from the downstream aeration cells. It should also beunderlined that the time intervals between the aeration cycles arelonger during the diurnal phases.

As in the process of regulation which is the subject of the alreadycited French patent, the effects of a dilution or of a pollutionoverload are reduced by the adaptation of the aeration cycles. Inparticular, the invention allows a reduction in the variations of thepolluting loads on upstream aeration cells. A dilution of the pollutingload, resulting for example from a downpour, causes an increase in thefrequency of the aeration cycles, both on the upstream and downstreamaeration cells. By contrast, a massive pollution overload limits theincrease in the values of the oxidation-reduction potential and theaeration cycles are then more prolonged and fewer in frequency; in thelatter case, the principal modification of aeration occurs on theupstream cells which then treat a substantial part of the pollutingload, reducing the disturbances and leaving the downstream cells tofinish the treatment.

This FIG. 5 illustrates the effects of a modification of the pollutionregime on the aeration sequence. In the example of application to whichFIG. 5 refers, a typical urban effluent, to which a substantial fractionof agro-industrial effluent has been added, is treated during theworking days. This FIG. 5 represents the aeration cycles over 48 hours,with respectively small and high loads (corresponding respectively to aSunday and to a Monday). The variation in the oxidation-reduction cycleson the upstream cells is modified with a high reduction in the slopes ofincrease during the working day. A high load is then visible, on Monday,from 10 am to 9 pm, whereas it was limited from 1 pm to 6 pm in the caseof Sunday. It is also observed that the lengths of the cycles have beenincreased linked to the carbon pollution load. On the downstreamaeration cells, it is important to note the modification of the envelopeof the variations in the oxidation-reduction potential between the twodays. The variation in amplitude is high on Sunday, when the low supplyof pollution is essentially treated on the upstream aeration cells. Bycontrast, when the treatment capacity of the upstream cells is exceeded,the downstream cells are subjected to higher loads than they shouldtreat. This gives the downstream cells a complementary character whenthe upstream aeration volumes are limited. Thus, the aeration is againadjusted to the polluting load. The energy expenditure necessary for theaeration process consequently tends to be adapted to the influentpolluting load.

The exemplary embodiment to which FIG. 5 refers is a unit of 40,000pop.eq. (population equivalent), which will be designated below site A,having a U-shaped aeration basin configuration as illustrated by FIG.2b, comprising two aeration cells of 2600 m³ each, the oxygenation beingprovided by conventional aeration turbines. The cumulative aeration perday, was about 5 h 30 min on the upstream cell and 4 h 30 min on thedownstream cell, for a respective aeration power of 120 kW and 80 kW,and a daily average load of 1050 kg of BOD. Even in differentconfigurations, the distribution of the aeration between upstream anddownstream cells is generally situated in the 60% (upstream)/40%downstream ratio. These results confirm that most of the treatment, asregards the degradation of the carbon compounds, is carried out on theupstream cell.

There will now be given, by way of nonlimiting examples, resultsobtained using the process according to the invention, on differentwater treatment plants.

The experiment reported here was carried out on three water treatmentplants all having a configuration of aeration basins in series. Theseplants, called hereinafter sites A, B, C, had a capacity of 40,000,15,000 and 14,000 pop.eq., respectively. On each site, a comparison wasmade between the result of the treatment with automatic regulationaccording to the invention and the result without regulation. Table 1below summarizes the comparative results thus obtained.

TABLE Site A Site A Site B Site B Site C Site C (without (with (without(with (without (with regu- regu- regu- regu- regu- regu- laton) lation)lation) lation) lation) lation) Compliance Yes Yes No Yes Yes Yes withEU standards C removal 98.0 98.8 97 98 98.9 99.0 yield (%) N removal95.1 95.1 50 80 88.8 93.8 yield (%) Energy 15% 5% 20% savings

The results thus mentioned clearly show that the system of regulationaccording to the invention makes it possible to comply with thedischarge standards laid down by the European Union.

As regards site B, it will be noted that without the regulationaccording to the invention, a discharge of a nonoxidized form of thenitrogen compounds is produced whereas by using the invention, theremoval of the nitrogen passes from 50 to 80%. The nitrogen compoundsare, in this case, completely oxidized.

As regards sites A and C, the carbon and nitrogen removal yields weremaintained or improved, these yields corresponding to the maximum whichcan be obtained by the capacity of the plant, even before introducingthe process according to the invention.

It will be noted, in addition, that the use of the process of regulationaccording to the present invention makes it possible to make substantialenergy savings.

The result of reading the preceding description is that the processwhich is the subject of the present invention provides particularlysatisfactory results. As previously explained, the upstream aerationcells bring about a treatment of the major part of the polluting load,with a priority given to the removal of the carbon and the downstreamcells complete the treatment by carrying out, moreover, the removal ofthe nitrogen. The invention makes it possible to obtain an excellentquality of the treated water accompanied by a high reliabilityespecially as regards the management and the optimization of theaeration process. Furthermore, the invention makes it possible to obtainan optimization of the energy consumption which results in substantialexploitation savings.

It of course remains the case that the present invention is not limitedto the exemplary embodiments described and/or represented but that itcovers all the variants which come within the scope of the appendedclaims.

What is claimed is:
 1. Process for regulating aeration of a biologicalwastewater treatment plant, for a purpose of removing carbon andnitrogen pollution, having aeration basins arranged in series, thisprocess comprising the following stages: an oxidation-reductionpotential EH is measured separately on each of the basins; themeasurements are respectively transmitted to a data processing systemfor a purpose of analyzing a variation of the oxidation-reductionpotential and diagnosis of capacities of an aeration-biology pair tobring about reactions degrading the carbon and nitrogen pollution, andstarting, continuing, or stopping aeration of the basins is determined,from the diagnosis, as a function of a desired degree of purificationsuch that: in first basins, or upstream basins, priority is given totreatment of the carbon pollution with a specific parameterizing ofthreshold values of the oxidation-reduction potential, and in lastbasins, or downstream basins, treatment of the carbon and nitrogenpollution is refined by a management of the aeration which consists inestablishing a curve of the variation of the oxidation-reductionpotential, measured on the downstream basins, as a function of time andin calculating a derivative and, when this derivative tends toward zero,the derivative and a value of the oxidation-reduction potential arecorrelated in order to determine the starting, the continuation or thestopping of the aeration of the downstream basins.
 2. Process accordingto claim 1, wherein, after establishment of the diagnosis, optimumperiods of operation or stoppage of aeration systems of different basinsare determined as a function of desired degree of purification, theperiods further comprising: period of aeration necessary toreequilibrate the systems in order to reestablish a satisfactory degreeof oxidation of sludge; period of aeration necessary for an oxygensupply which allows best use of reactions for degrading the carbon loadin the upstream basin(s); period of aeration necessary to complete asupply of oxygen in order to optimize the reactions for degrading thecarbon load and nitrification in the downstream basin(s); periodnecessary for stopping aeration in order to complete denitrification inthe downstream basin(s).
 3. Process according to claim 1, furtherwherein warning or defect signals are generated which relate inparticular to deficiencies in an aeration system, a permanent ortemporary polluting overload, a deterioration of a biomass, defects inan operation of probes for measuring oxidation-reduction potential,hydraulic interactions between the different basins.
 4. Processaccording t o claim 2, wherein warning or defect signals are generatedwhich relate in particular to deficiencies in aeration systems, apermanent or temporary polluting overload, a deterioration of a biomass,defects in an operation of probes for measuring the oxidation-reductionpotential, hydraulic interactions between the different basins.